mbe: Handle applying macro_rules derives

Add infrastructure to apply a derive macro to arguments, consuming and
returning a `TokenTree` only.

Handle `SyntaxExtensionKind::MacroRules` when expanding a derive, if the
macro's kinds support derive.

Add tests covering various cases of `macro_rules` derives.

Author: joshtriplett

compiler/rustc_expand/src/expand.rs

@@ -26,7 +26,7 @@ use rustc_session::lint::BuiltinLintDiag;
 use rustc_session::lint::builtin::{UNUSED_ATTRIBUTES, UNUSED_DOC_COMMENTS};
 use rustc_session::parse::feature_err;
 use rustc_session::{Limit, Session};
-use rustc_span::hygiene::SyntaxContext;
+use rustc_span::hygiene::{MacroKinds, SyntaxContext};
 use rustc_span::{ErrorGuaranteed, FileName, Ident, LocalExpnId, Span, Symbol, sym};
 use smallvec::SmallVec;
 
@@ -923,6 +923,35 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                     }
                     fragment
                 }
+                SyntaxExtensionKind::MacroRules(expander)
+                    if expander.kinds().contains(MacroKinds::DERIVE) =>
+                {
+                    if is_const {
+                        let guar = self
+                            .cx
+                            .dcx()
+                            .span_err(span, "macro `derive` does not support const derives");
+                        return ExpandResult::Ready(fragment_kind.dummy(span, guar));
+                    }
+                    let body = item.to_tokens();
+                    match expander.expand_derive(self.cx, span, &body) {
+                        Ok(tok_result) => {
+                            let fragment =
+                                self.parse_ast_fragment(tok_result, fragment_kind, &path, span);
+                            if macro_stats {
+                                update_derive_macro_stats(
+                                    self.cx,
+                                    fragment_kind,
+                                    span,
+                                    &path,
+                                    &fragment,
+                                );
+                            }
+                            fragment
+                        }
+                        Err(guar) => return ExpandResult::Ready(fragment_kind.dummy(span, guar)),
+                    }
+                }
                 _ => unreachable!(),
             },
             InvocationKind::GlobDelegation { item, of_trait } => {

compiler/rustc_expand/src/mbe/diagnostics.rs

@@ -14,14 +14,22 @@ use super::macro_rules::{MacroRule, NoopTracker, parser_from_cx};
 use crate::expand::{AstFragmentKind, parse_ast_fragment};
 use crate::mbe::macro_parser::ParseResult::*;
 use crate::mbe::macro_parser::{MatcherLoc, NamedParseResult, TtParser};
-use crate::mbe::macro_rules::{Tracker, try_match_macro, try_match_macro_attr};
+use crate::mbe::macro_rules::{
+    Tracker, try_match_macro, try_match_macro_attr, try_match_macro_derive,
+};
+
+pub(super) enum FailedMacro<'a> {
+    Func,
+    Attr(&'a TokenStream),
+    Derive,
+}
 
 pub(super) fn failed_to_match_macro(
     psess: &ParseSess,
     sp: Span,
     def_span: Span,
     name: Ident,
-    attr_args: Option<&TokenStream>,
+    args: FailedMacro<'_>,
     body: &TokenStream,
     rules: &[MacroRule],
 ) -> (Span, ErrorGuaranteed) {
@@ -36,10 +44,12 @@ pub(super) fn failed_to_match_macro(
     // diagnostics.
     let mut tracker = CollectTrackerAndEmitter::new(psess.dcx(), sp);
 
-    let try_success_result = if let Some(attr_args) = attr_args {
-        try_match_macro_attr(psess, name, attr_args, body, rules, &mut tracker)
-    } else {
-        try_match_macro(psess, name, body, rules, &mut tracker)
+    let try_success_result = match args {
+        FailedMacro::Func => try_match_macro(psess, name, body, rules, &mut tracker),
+        FailedMacro::Attr(attr_args) => {
+            try_match_macro_attr(psess, name, attr_args, body, rules, &mut tracker)
+        }
+        FailedMacro::Derive => try_match_macro_derive(psess, name, body, rules, &mut tracker),
     };
 
     if try_success_result.is_ok() {
@@ -90,7 +100,7 @@ pub(super) fn failed_to_match_macro(
     }
 
     // Check whether there's a missing comma in this macro call, like `println!("{}" a);`
-    if attr_args.is_none()
+    if let FailedMacro::Func = args
         && let Some((body, comma_span)) = body.add_comma()
     {
         for rule in rules {

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -29,7 +29,7 @@ use rustc_span::hygiene::{MacroKinds, Transparency};
 use rustc_span::{Ident, Span, Symbol, kw, sym};
 use tracing::{debug, instrument, trace, trace_span};
 
-use super::diagnostics::failed_to_match_macro;
+use super::diagnostics::{FailedMacro, failed_to_match_macro};
 use super::macro_parser::{NamedMatches, NamedParseResult};
 use super::{SequenceRepetition, diagnostics};
 use crate::base::{
@@ -138,7 +138,6 @@ pub(super) enum MacroRule {
         rhs: mbe::TokenTree,
     },
     /// A derive rule, for use with `#[m]`
-    #[expect(unused)]
     Derive { body: Vec<MatcherLoc>, body_span: Span, rhs: mbe::TokenTree },
 }
 
@@ -167,6 +166,63 @@ impl MacroRulesMacroExpander {
     pub fn kinds(&self) -> MacroKinds {
         self.kinds
     }
+
+    pub fn expand_derive(
+        &self,
+        cx: &mut ExtCtxt<'_>,
+        sp: Span,
+        body: &TokenStream,
+    ) -> Result<TokenStream, ErrorGuaranteed> {
+        // This is similar to `expand_macro`, but they have very different signatures, and will
+        // diverge further once derives support arguments.
+        let Self { name, ref rules, node_id, .. } = *self;
+        let psess = &cx.sess.psess;
+
+        if cx.trace_macros() {
+            let msg = format!("expanding `#[derive({name})] {}`", pprust::tts_to_string(body));
+            trace_macros_note(&mut cx.expansions, sp, msg);
+        }
+
+        match try_match_macro_derive(psess, name, body, rules, &mut NoopTracker) {
+            Ok((rule_index, rule, named_matches)) => {
+                let MacroRule::Derive { rhs, .. } = rule else {
+                    panic!("try_match_macro_derive returned non-derive rule");
+                };
+                let mbe::TokenTree::Delimited(rhs_span, _, rhs) = rhs else {
+                    cx.dcx().span_bug(sp, "malformed macro derive rhs");
+                };
+
+                let id = cx.current_expansion.id;
+                let tts = transcribe(psess, &named_matches, rhs, *rhs_span, self.transparency, id)
+                    .map_err(|e| e.emit())?;
+
+                if cx.trace_macros() {
+                    let msg = format!("to `{}`", pprust::tts_to_string(&tts));
+                    trace_macros_note(&mut cx.expansions, sp, msg);
+                }
+
+                if is_defined_in_current_crate(node_id) {
+                    cx.resolver.record_macro_rule_usage(node_id, rule_index);
+                }
+
+                Ok(tts)
+            }
+            Err(CanRetry::No(guar)) => Err(guar),
+            Err(CanRetry::Yes) => {
+                let (_, guar) = failed_to_match_macro(
+                    cx.psess(),
+                    sp,
+                    self.span,
+                    name,
+                    FailedMacro::Derive,
+                    body,
+                    rules,
+                );
+                cx.macro_error_and_trace_macros_diag();
+                Err(guar)
+            }
+        }
+    }
 }
 
 impl TTMacroExpander for MacroRulesMacroExpander {
@@ -328,8 +384,15 @@ fn expand_macro<'cx>(
         }
         Err(CanRetry::Yes) => {
             // Retry and emit a better error.
-            let (span, guar) =
-                failed_to_match_macro(cx.psess(), sp, def_span, name, None, &arg, rules);
+            let (span, guar) = failed_to_match_macro(
+                cx.psess(),
+                sp,
+                def_span,
+                name,
+                FailedMacro::Func,
+                &arg,
+                rules,
+            );
             cx.macro_error_and_trace_macros_diag();
             DummyResult::any(span, guar)
         }
@@ -391,8 +454,15 @@ fn expand_macro_attr(
         Err(CanRetry::No(guar)) => Err(guar),
         Err(CanRetry::Yes) => {
             // Retry and emit a better error.
-            let (_, guar) =
-                failed_to_match_macro(cx.psess(), sp, def_span, name, Some(&args), &body, rules);
+            let (_, guar) = failed_to_match_macro(
+                cx.psess(),
+                sp,
+                def_span,
+                name,
+                FailedMacro::Attr(&args),
+                &body,
+                rules,
+            );
             cx.trace_macros_diag();
             Err(guar)
         }
@@ -539,6 +609,44 @@ pub(super) fn try_match_macro_attr<'matcher, T: Tracker<'matcher>>(
     Err(CanRetry::Yes)
 }
 
+/// Try expanding the macro derive. Returns the index of the successful arm and its
+/// named_matches if it was successful, and nothing if it failed. On failure, it's the caller's job
+/// to use `track` accordingly to record all errors correctly.
+#[instrument(level = "debug", skip(psess, body, rules, track), fields(tracking = %T::description()))]
+pub(super) fn try_match_macro_derive<'matcher, T: Tracker<'matcher>>(
+    psess: &ParseSess,
+    name: Ident,
+    body: &TokenStream,
+    rules: &'matcher [MacroRule],
+    track: &mut T,
+) -> Result<(usize, &'matcher MacroRule, NamedMatches), CanRetry> {
+    // This uses the same strategy as `try_match_macro`
+    let body_parser = parser_from_cx(psess, body.clone(), T::recovery());
+    let mut tt_parser = TtParser::new(name);
+    for (i, rule) in rules.iter().enumerate() {
+        let MacroRule::Derive { body, .. } = rule else { continue };
+
+        let mut gated_spans_snapshot = mem::take(&mut *psess.gated_spans.spans.borrow_mut());
+
+        let result = tt_parser.parse_tt(&mut Cow::Borrowed(&body_parser), body, track);
+        track.after_arm(true, &result);
+
+        match result {
+            Success(named_matches) => {
+                psess.gated_spans.merge(gated_spans_snapshot);
+                return Ok((i, rule, named_matches));
+            }
+            Failure(_) => {
+                mem::swap(&mut gated_spans_snapshot, &mut psess.gated_spans.spans.borrow_mut())
+            }
+            Error(_, _) => return Err(CanRetry::Yes),
+            ErrorReported(guar) => return Err(CanRetry::No(guar)),
+        }
+    }
+
+    Err(CanRetry::Yes)
+}
+
 /// Converts a macro item into a syntax extension.
 pub fn compile_declarative_macro(
     sess: &Session,

compiler/rustc_resolve/messages.ftl

@@ -249,7 +249,7 @@ resolve_macro_cannot_use_as_attr =
     `{$ident}` exists, but has no `attr` rules
 
 resolve_macro_cannot_use_as_derive =
-     `{$ident}` exists, but a declarative macro cannot be used as a derive macro
+     `{$ident}` exists, but has no `derive` rules
 
 resolve_macro_defined_later =
     a macro with the same name exists, but it appears later

tests/ui/macros/macro-rules-as-derive-or-attr-issue-132928.stderr

@@ -2,7 +2,7 @@ error: cannot find derive macro `sample` in this scope
   --> $DIR/macro-rules-as-derive-or-attr-issue-132928.rs:6:10
    |
 LL | macro_rules! sample { () => {} }
-   |              ------ `sample` exists, but a declarative macro cannot be used as a derive macro
+   |              ------ `sample` exists, but has no `derive` rules
 ...
 LL | #[derive(sample)]
    |          ^^^^^^
@@ -20,7 +20,7 @@ error: cannot find derive macro `sample` in this scope
   --> $DIR/macro-rules-as-derive-or-attr-issue-132928.rs:6:10
    |
 LL | macro_rules! sample { () => {} }
-   |              ------ `sample` exists, but a declarative macro cannot be used as a derive macro
+   |              ------ `sample` exists, but has no `derive` rules
 ...
 LL | #[derive(sample)]
    |          ^^^^^^
@@ -31,7 +31,7 @@ error: cannot find derive macro `sample` in this scope
   --> $DIR/macro-rules-as-derive-or-attr-issue-132928.rs:6:10
    |
 LL | macro_rules! sample { () => {} }
-   |              ------ `sample` exists, but a declarative macro cannot be used as a derive macro
+   |              ------ `sample` exists, but has no `derive` rules
 ...
 LL | #[derive(sample)]
    |          ^^^^^^

tests/ui/macros/macro-rules-derive-error.rs

@@ -0,0 +1,44 @@
+#![feature(macro_derive)]
+
+macro_rules! MyDerive {
+    derive() { $($body:tt)* } => {
+        compile_error!(concat!("MyDerive: ", stringify!($($body)*)));
+    };
+    //~^^ ERROR: MyDerive
+}
+
+//~v NOTE: `fn_only` exists, but has no `derive` rules
+macro_rules! fn_only {
+    {} => {}
+}
+
+//~v NOTE: `DeriveOnly` exists, but has no rules for function-like invocation
+macro_rules! DeriveOnly {
+    derive() {} => {}
+}
+
+fn main() {
+    //~v NOTE: in this expansion of #[derive(MyDerive)]
+    #[derive(MyDerive)]
+    struct S1;
+
+    //~vv ERROR: cannot find macro `MyDerive` in this scope
+    //~| NOTE: `MyDerive` is in scope, but it is a derive
+    MyDerive!(arg);
+
+    //~v ERROR: cannot find derive macro `fn_only` in this scope
+    #[derive(fn_only)]
+    struct S2;
+
+    DeriveOnly!(); //~ ERROR: cannot find macro `DeriveOnly` in this scope
+}
+
+//~vv ERROR: cannot find derive macro `ForwardReferencedDerive` in this scope
+//~| NOTE: consider moving the definition of `ForwardReferencedDerive` before this call
+#[derive(ForwardReferencedDerive)]
+struct S;
+
+//~v NOTE: a macro with the same name exists, but it appears later
+macro_rules! ForwardReferencedDerive {
+    derive() {} => {}
+}

tests/ui/macros/macro-rules-derive-error.stderr

@@ -0,0 +1,51 @@
+error: MyDerive: struct S1;
+  --> $DIR/macro-rules-derive-error.rs:5:9
+   |
+LL |         compile_error!(concat!("MyDerive: ", stringify!($($body)*)));
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+...
+LL |     #[derive(MyDerive)]
+   |              -------- in this derive macro expansion
+   |
+   = note: this error originates in the derive macro `MyDerive` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: cannot find macro `MyDerive` in this scope
+  --> $DIR/macro-rules-derive-error.rs:27:5
+   |
+LL |     MyDerive!(arg);
+   |     ^^^^^^^^
+   |
+   = note: `MyDerive` is in scope, but it is a derive macro: `#[derive(MyDerive)]`
+
+error: cannot find derive macro `fn_only` in this scope
+  --> $DIR/macro-rules-derive-error.rs:30:14
+   |
+LL | macro_rules! fn_only {
+   |              ------- `fn_only` exists, but has no `derive` rules
+...
+LL |     #[derive(fn_only)]
+   |              ^^^^^^^
+
+error: cannot find macro `DeriveOnly` in this scope
+  --> $DIR/macro-rules-derive-error.rs:33:5
+   |
+LL | macro_rules! DeriveOnly {
+   |              ---------- `DeriveOnly` exists, but has no rules for function-like invocation
+...
+LL |     DeriveOnly!();
+   |     ^^^^^^^^^^
+
+error: cannot find derive macro `ForwardReferencedDerive` in this scope
+  --> $DIR/macro-rules-derive-error.rs:38:10
+   |
+LL | #[derive(ForwardReferencedDerive)]
+   |          ^^^^^^^^^^^^^^^^^^^^^^^ consider moving the definition of `ForwardReferencedDerive` before this call
+   |
+note: a macro with the same name exists, but it appears later
+  --> $DIR/macro-rules-derive-error.rs:42:14
+   |
+LL | macro_rules! ForwardReferencedDerive {
+   |              ^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 5 previous errors
+